As brake systems for a vehicle, hydraulic brake systems have been used in which a friction pad is pressed against a brake disk by a hydraulic cylinder so as to generate braking force. However, in recent years, with the introduction of brake control such as anti-lock brake system (ABS), electric brake systems in which no hydraulic circuit is used are gathering attention.
Electric brake systems include an electric linear motion actuator provided with an electric motor as a driving source, and a friction pad is pressed against a brake disk by this electric linear motion actuator so as to generate braking force.
As such an electric brake system, the electric brake system, for example, disclosed in Japanese Application Publication 2011-241851 is known. This publication discloses an electric linear motion actuator by which a friction pad is pressed against a brake disk. This electric linear motion actuator includes an electric motor, a motion conversion mechanism for converting the torque generated in the electric motor to the linear driving force of a linear motion member such that the friction pad is pressed against the brake disk by the linear motion member, and a load sensor configured to detect the magnitude of the pressing force applied to the brake disk. The torque of the electric motor is controlled on the basis of the magnitude of the pressing force detected by the load sensor.
If the electric linear motion actuator of the JP 2011-241851 publication is used, by continuing to apply constant electric current to the electric motor, it is possible to maintain the pressing force applied to the brake disk from the linear motion member. If the electric current applied to the electric motor at this time can be reduced, it is possible to effectively reduce the power consumption of the electric motor.
In a motion conversion mechanism for converting the torque of an electric motor to the linear driving force of a linear motion member, and a pressing force is applied to a target member by the linear motion member, large frictional forces occur in the motion conversion mechanism due to the reaction force which acts on the linear motion member from the target member. Such frictional forces cause energy loss, and thus are generally undesirable.